In this paper, a new online monitoring method is developed to determine the moisture change during the vacuum drying of lithium ion battery powder. Firstly, a mathematical model is proposed according to the relevant thermodynamic formulas. Then a series of experiments are carried out under different drying conditions to assess the feasibility and reliability of the novel monitoring method. The relative humidity and temperature at pump outlet are recorded for the judgment of drying endpoint and the prediction of moisture content. The uncertainty analysis of novel method is carried out at the end of this paper. The results show that the novel method has the advantages of low cost, easy operation and real-time response in detecting the moisture change. Moreover, the novel method can estimate the moisture content accurately so the predicted value can be used as a reference for optimizing the drying process. (C) 2020 Elsevier B.V. All rights reserved.\